Wavelength-division multiplexing communication systems are widely applied to communication networks such as a trunk system to deal with increasing network traffic due to rapid growth of the Internet. In the wavelength-division multiplexing communication system, a plurality of optical signals of different wavelengths from each other is simultaneously transmitted on a single optical fiber.
Moreover, a wavelength-division multiplexed-passive optical subscriber network (hereinafter, referred to as a WDM-PON) is characterized in that bi-directional symmetrical service and excellent security are guaranteed since optical signals of discrete wavelengths, e.g., upstream optical signals of about 1.3 μm band (1260˜1360 nm) and downstream optical signals of about 1.55 μm band (1480˜1580 nm) are transmitted between an optical line terminal (hereinafter, referred to as an OLT) or central office (CO) and each optical network unit (hereinafter, referred to as an ONU)
FIG. 1 shows a schematic diagram of an exemplary WDM-PON of a prior art. As shown in FIG. 1, the WDM-PON includes an OLT a 10, an RN 20, an ONU 30 and a single optical fiber 15 for transmitting upstream and downstream signals between the OLT 10 and the ONU 30.
The OLT 10 includes a plurality set of an optical source 11 and a variable optical attenuators/polarization controller. (VOA/PC) 12 and a multiplexer (MUX) 14. The optical sources 11 are made up of laser diodes, e.g., LD1, LD2, . . . , LD16, which oscillate discrete wavelengths λ1, λ2, . . . , λ16, respectively, and generate optical signals by modulating the discrete wavelengths. The multiplexer 14 multiplexes the optical signals of discrete wavelengths to route the multiplexed optical signals to the optical fiber 15. The variable optical attenuators/polarization controllers 12 are disposed between the optical sources 11 and the multiplexer 14 to perform a function of uniformly adjusting each power of the optical signals before loading the optical signals of different wavelengths on the optical fiber 15.
The RN 20 has a demultiplexer (DEMUX) 20, embedded as a waveguide grating router, for separating the multiplexed optical signals from the OLT 10 via the optical fiber 15 by each discrete wavelength.
The ONU 30 includes a plurality set of a variable optical attenuator/polarization controller (VOA/PC) 32, a band pass filter (BPF) 34 and an optical receiver 36. The optical receivers 36 are made up of photo diodes PD1, PD2 . . . PD16, respectively, each detecting an optical signal separated by the demultiplexer 20. The variable optical attenuators/polarization controllers 32 are disposed between the demultiplexer 20 and the photo detectors 36 to adjust the powers of optical signals transmitted from the OLT 10, respectively. The band pass filters 24 adjust the optical signals pursuant to a data rate.
The WDM-PON further includes optical amplifiers (OAs) 16 and 19 for compensating losses caused when transmitting the optical signals between the multiplexer 14 and the demultiplexer 20 via the optical fiber 15, and a dispersion compensation fiber (DCF) 18 for compensating color dispersions of the optical signals, accumulated during a long distance transmission.
In the WDM-PON, for downstream transmission of the optical signals, the optical signals of discrete wavelengths are generated in the respective optical sources 11. The downstream optical signals pass their respective corresponding variable optical attenuators/polarization controllers 12 to be routed to the multiplexer 14. The multiplexer 14 multiplexes the downstream optical signals and routes the multiplexed optical signals to the RN 20 via the optical fiber 15. At this time, optical losses and color dispersions of the multiplexed optical signals are compensated through the optical amplifiers 16, 19 and the dispersion compensation fiber 18.
The demultiplexer 20 in the RN 20 separates the multiplexed optical signals by each discrete wavelength and routes the separated optical signals to the ONU (ONU). In the ONU, the downstream optical signals are detected through the corresponding optical receivers 36 (PD1, PD2 . . . PD16) via the variable optical attenuators/polarization controllers 32 and the band pass filters 34, respectively.
On the other hand, upstream transmission is opposite to the aforementioned downstream transmission and is easily known by those skilled in the art. Therefore, the detailed description thereof is omitted for the sake of simplicity of the description.
In the WDM-PON as described above, in order to transmit the upstream and downstream optical signals between the OLT and the ONUs, crosstalk between adjacent channels has to be large; an optical power of optical signal has to be large; a line width is small; and influence on color dispersion has to be small. Thus, a high-priced, high power and broad band optical source such as a light emitting diode (LED), a super luminescent diode and so forth should be used for overcoming a loss by a distance from the OLT to the ONUS.
Recently, there has been research on a wavelength division low-priced optical source, which employs a Fabry-Perot laser diode. However, the Fabry-Perot laser diode has drawbacks in that a mode hopping and a mode partition are appeared and a wavelength displacement depending on a temperature variation is large. In order to overcome these drawbacks, there has been research on a wavelength locked Fabry-Perot laser diode by a non-interfered light. However, a high-priced broad band optical source having a higher power has to be additionally installed in the OLT, and a plurality of circulators has to be needed.
A distributed feedback laser diode is now employed as a wavelength division optical source for an optical communication of a high speed and a high power, which satisfies the above-mentioned requisites. This distributed feedback laser diode is adequate for a high speed and long-distance signal transmission resulting from a narrow line width, but it is high-priced. Thus, in case of applying the distributed feedback laser diode to a PON, it is needed to equip a plurality of the distributed feedback laser diodes corresponding to different wavelengths from each other assigned to the ONUs. As a result, cost of the PON is increased. Therefore, it is necessary to furnish a low-cost optical source.
Further, in the WOM-PON, the waveguide grating router for splitting optical signals by discrete wavelengths has to be provided with an additional device, which carries out remotely monitoring of the fluctuation of a pass wavelength depending on a temperature variation in the remote node One of methods monitoring a temperature variation of the waveguide grating router in the above-described WDM-PON is disclosed in the paper of S. Hann, D. H. Kim and C. S. Park, “Uni-lambda bidirection 10/1.25 GbE access service based on WDM-PON”, Electron. Lett., Vol. 40. No. 3, pp 194-195, 5 Feb. 2004. In addition, there is the paper of R. Giles, S. Jiang, “Fiber-grating sensor for wavelength tracking in single-fiber WDM access PONs”, IEEE Photon. Technol. Lett., vol 9, pp 523-525, April, 1997.
Furthermore, a lot of researches on the waveguide grating router unrelated to a temperature variation has been done. However, a lot of cost is required in manufacturing the waveguide grating router, and there remains the problem to be solved yet.
Therefore, there is a need to provide a WDM-PON tuned to a variation of a pass band wavelength depending on a temperature variation of the waveguide grating router while using a low-priced optical source without a temperature monitoring.